There are really two concerns here.
- the replacement meal should not harm the cat
- the replacement meal should be palatable so that the cat will eat it
The danger in meal replacements is the loss of taurine that is added to commercial cat food that cats need to survive. According to VCA Animal Hospitals:
Taurine is exclusively found in animal-based proteins. It is critical
for normal vision, normal digestion, normal heart muscle function, to
maintain normal pregnancy and fetal development, and to maintain a
healthy immune system. Most mammals are able to manufacture enough
taurine from other amino acids to meet their needs. However, cats have
a limited ability to manufacture taurine; therefore taurine is
classified as an essential nutrient in the cat. Fortunately for the
cat, taurine is readily obtained from the diet, as long as the diet
contains animal-based proteins. Unfortunately, it is not stored in
large quantities in the body and so must be consumed on a regular
Clinical signs of taurine deficiency are slow to develop. It can take
between five months and two years before symptoms become apparent,
depending on the cat's life stage.
If taurine levels are deficient, the retinal cells of the eyes will
eventually degenerate, impairing the vision. This condition is
referred to as feline central retinal degeneration (CRD). Deficiency
of taurine will also lead to a weakening of the muscle cells in the
heart, causing a condition called dilated cardiomyopathy. Taurine is a
component of bile salts, and its deficiency may cause digestive
(VCA owns/runs many of the emergency and specialist veterinarian hospitals in my area. I've been to 2 of their facilities and found them extremely competent).
For a few days a reduced supplement concentration (you're still feeding dry, just one meal is supplement-free) should be fine. If you were doing shift work or something and couldn't get to a pet store for longer than that, I would suggest looking into Amazon or other delivery options in your area.
After considering the safety from missing Taurine, I would also consider the potential dangers of the candidate materials. The most important ingredient is a meat source.
In the US, boiled chicken (no spices) and rice is often recommended by veterinarians for cats who have stomach upset. I would consider that safe.
Many people will give their cats cans of tuna, but that may not safe depending on your level of risk tolerance.
Beef (perhaps as of hamburger) is also common in US households, but if your cat does not already eat beef foods I would not recommend it (I've seen anaphylaxis in cats unaccustomed to beef before).
I suspect that this will be the real problem in suddenly changing your cat's food. I have never been able to get a cat to eat chicken and rice (even when they're feeling good, as an experiment to see if they didn't eat it before because they felt bad, or because they didn't like it).
There are pages and pages of strategies written on the web for how to switch your cat to raw/homemade food just because most cats (especially dry food fed cats) don't recognize it as food initially.
You can prepare the most healthy replacement, but if he doesn't think it's food, he won't eat it.
What I suspect the best strategy will be to think about any time he's shown interest in something that you've eaten (climbed in your lap, licked a dirty bowl, etc) and if that's something that's reasonable, make something like that.
For example, I had a cat who once showed interest in chicken wings. Boiled chicken with just a TINY dab of BBQ sauce (for smell/interest) might have worked on him. I had another cat who wanted string cheese. That's not a reasonable food.
If your cat has never shown interest in human food (or only inappropriate foods), just give him dry food for a few days and get to the store as quickly as you can.
If this is a recurrent problem, many pet food suppliers let you set up delivery subscriptions for pet foods so you won't forget (I do this! I recommend it!).